Conventionally, a technology to grow a nitride semiconductor crystal on a heterogeneous substrate such as sapphire is known (see e.g. PTLs 1 to 4).
According to PTL 1, after a base layer made of a nitride semiconductor is formed on a heterogeneous substrate, convexity and concavity are formed by etching the base layer through the heterogeneous substrate, and then the nitride semiconductor is grown on the base layer that has the convexity and concavity. In the concavity, a gap is formed between the heterogeneous substrate and the nitride semiconductor. When the electromagnetic wave is irradiated to the heterogeneous substrate in this situation, the heterogeneous substrate and the nitride semiconductor can be separated at interface therebetween.
According to PTLs 2, 3, after a base layer made of a nitride semiconductor is formed on a heterogeneous substrate, convexity and concavity are formed only on the base layer, and then the nitride semiconductor is grown on the base layer that has the convexity and concavity. In the concavity, a gap is formed between the heterogeneous substrate and the nitride semiconductor, and the nitride semiconductor is grown in the lateral direction and vertical direction. Thereby, a strain in the grown nitride semiconductor can be reduced.
According to PTL 4, after a base layer made of a nitride semiconductor is formed on a heterogeneous substrate, a groove is formed on the surface of the heterogeneous substrate that is exposed in a groove formed in the base layer. Then, the nitride semiconductor is grown on the base layer. A gap is formed between the heterogeneous substrate and the nitride semiconductor in a concavity. Since the groove is formed on the surface of the heterogeneous substrate, the heterogeneous substrate can be broken so as to reduce the strain in the nitride semiconductor when the stress is caused by a difference in thermal expansion coefficient between the heterogeneous substrate and the grown nitride semiconductor.